1. Field of the Invention
The present invention relates to a transporter vehicle such as a dump truck which is suitably used in transporting crushed stones excavated from such as an open-pit stope, a stone quarry, a mine, or dug earth and sand or the like.
2. Description of Prior Art
In general, a large-size transporter vehicle called a dump truck has a liftable vessel (loading platform) on a frame of a vehicle body, and carries and transports objects to be transported such as crushed stones or earth and sand or the like to an unloading site or a cargo collection site of, for instance, an exit port in a state in which the objects to be transported are loaded in a large quantity on the vessel (e.g., see Japanese Patent Laid-Open No. 2004-268646, Japanese Patent Laid-Open No. 2007-62697).
A transporter vehicle of this type according to the prior art is comprised of a vehicle body capable of self traveling; a loading platform which is provided on the vehicle body in such a manner as to be capable of being moved vertically (tilted) and on which the objects to be transported are loaded; a hoist cylinder which is telescopically provided between the loading platform and the vehicle body and in which a rod is extended at the time of discharging the objects to be transported from the loading platform so as to raise the loading platform diagonally backward with respect to the vehicle body; and a control valve unit which is provided between the hoist cylinder and a hydraulic power source to extend or contract the rod of the hoist cylinder, and which controls the supply and discharge of pressure oil to the hoist cylinder.
Further, the control valve unit used in such a transporter vehicle has a total of four changeover positions, including a raising position for raising the loading platform by extending the hoist cylinder by supplying and discharging the pressure oil from the hydraulic power source in one direction; a lowering position for lowering the loading platform downward by contracting the hoist cylinder by supplying and discharging the pressure oil in the other direction; a floating position for allowing the self-weight fall of the loading platform by contracting the hoist cylinder by the self-weight of the loading platform side; and a neutral position for stopping the movement of the hoist cylinder by stopping the supply and discharge of the pressure oil.
Here, the control valve unit is selectively changed over to one changeover position among the total of four changeover positions by manually performing the tilting operation of a remote control means such as an operating lever. Further, after the transporter vehicle traveled by itself to the cargo collection site in the state in which the earth and sand or crushed stones or the like are loaded on the loading platform, the rod of the hoist cylinder is extended to raise the loading platform diagonally backward. Through this raising operation, the earth and sand or crushed stones or the like are discharged to the cargo collection site along the tilting direction of the loading platform.
By the way, in the above-described transporter vehicle according to the prior art, the control valve unit can be changed over to one of the four changeover positions in accordance with the remote control of the operating lever or the like by driving a main hydraulic pump, a pilot pump and the like by an engine serving as a prime mover, thereby making it possible to control the extending or contracting motion of the hoist cylinder for moving the loading platform on the vehicle body.
In contrast, when the engine is stopped, since the hydraulic pump, the pilot pump, and the like are also stopped, the control valve unit cannot be changed over by the remote control. However, at the time of performing maintenance, inspection, and the like of the transporter vehicle, there are cases where, for example, by stopping the engine with the loading platform kept raised diagonally backward, maintenance work is hence performed in the state in which the noise due to the engine sound is eliminated while ensuring the safety of the operator.
For this reason, according to the prior art, the engine is started again upon completion of the maintenance work, and in this state, the hoist cylinder is contracted by the self-weight on the loading platform side by changing over the control valve unit to, for example, the floating position, and the engine is stopped when the loading platform is seated onto the vehicle body.
However, there are problems in that the temporary operation of the engine for the sole purpose of lowering the loading platform is wasteful from the viewpoint of energy saving, and it also constitutes an extra burden on the operator.